Communication networks are becoming widely used for distributing both high and low speed data over varying distances. Typically, a communication network is comprised of a number of network elements (NE) that are connected to each other in a variety of configurations so as to form a unified communication network. The communication network may extend over a small area, such as a company wide network, or may cover large distances, such as in regional or nationwide networks. The NE's allow network clients to input data for transmission over the network and to receive data transmitted over the network from other locations. Thus, data may be added to, or dropped from the network at NE locations, as the data flows from point to point throughout the network.
One problem associated with communication networks is the problem of protection switching. In one situation, protection switching involves the process of switching network traffic from one network transmission path to another in the event of a network failure. For example, in a ring network, network traffic moves around the ring from network element to network element. In the event of a network failure, for example, a failure where a network link is damaged or broken, network traffic is switched so that is it routed away from the failure. Thus, traffic that is initially flowing in a clockwise manner around the ring is switched to flow in a counterclockwise direction that is away from the failure.
In another protection switching technique, typically used in ring networks, the network traffic is transmitted over working and protection transmission paths that flow in different directions over the network to the same destination. In the event of a network failure, either the working or protection transmission path will be selected to deliver the network traffic to the network element at the final destination.
FIG. 1 shows a typical protection switching arrangement in a network element 100 that includes a pair of line cards 102, 104 that are coupled to a communication network. Assuming the network traffic is bridged, line card 102 receives working traffic and line card 104 receives protection traffic. Typically, high-speed network transmissions include a number of low speed channels, which the line cards extract from the working and protection transmissions. A channel output from the two line cards is cross-coupled to a pair of switch cards 106, 108. The switch cards output one of the inputs they receive to an output device 110 for output to a local user's equipment 112.
The switch cards 106, 108 include selection logic on each card to determine which of the traffic signals at their input will be provided at their respective output. Thus, each switch card makes it own determination whether to provide a channel from the working or protection traffic to the output device. Lastly, the output device determines from the traffic signals it receives, which signal will be output to the local user.
The arrangement described above may be used as part of a synchronous optical network (SONET) and is commonly referred to as a cross-connected SONET protection system. In a complete implementation, redundant switch cards, identical to switch cards 106, 108, are provided to switch each of the channels in the network traffic upon the occurrence of a network fault. Unfortunately, this means that it is necessary to have a separate switch matrix for each channel in the network transmission, where each switch matrix is made up of two separate switch cards coupled to the individual line cards, respectively. This type of arrangement has the disadvantages of complexity and high cost, since it requires several different types of circuits. Furthermore, when a network fault occurs, it is necessary for multiple switching events to occur to switch all of the traffic, where the switching events happen independently from each other. As a result, the system lacks efficiency.
Another typical implementation for protection switching in a network includes a centralized processor performing protection switching using a simple selection algorithm. However, such an implementation is not tolerant to failure of the centralized processor. In addition, a centralized processor controlling several independent peripheral pieces of switching hardware must communicate independently with each of them, thereby increasing the complexity of the system.
Therefore, it would be desirable to have a system to perform protection switching in a communication network that does not involve a centralized processor and that does not require complex, independent switching circuits that are costly and inefficient.